Field effect transistors are known to have a structure of an electronic element taking advantage of a charge separation phenomenon of a zinc oxide laminate (for example, patent document 1). These semiconductors serve as elements having a switching function by taking advantage of changes of the charge separation states generated by applying an electric field. Such electronic elements are used for switching of display devices. Electrons are attracted into an area called a channel in a field effect transistor by applying an electric field to the element through an electrode called a gate. The transistor is used in an ON-state by permitting a channel portion to be a high electron density area, and it is used in an OFF-state when electrons are not attracted. On the contrary, it is possible to provide the OFF-state by sweeping the electrons in the channel area by applying a voltage on the gate.
Different from the field effect transistor described above, a modulated doping method is known for permitting charge separation to occur by allowing semiconductors to contact one another while no electric field is applied (for example, non-patent document 1). Charge transfer from a semiconductor having a high electron density to a semiconductor having high electron mobility is induced by laminating a semiconductor having a wide band gap and high electron density with a semiconductor having a narrow band gap and high electron mobility. Consequently, a semiconductor material that satisfies both the high electron density and high electron mobility is formed by allowing electrons to move through a layer having high mobility.
The transistor having high electron mobility is an electronic element obtainable by joining semiconductor materials having different band gaps and electron densities one another, wherein electrons are injected from a carrier-supplying layer having an intrinsically high electron density to a carrier-transfer layer having an intrinsically low electron density. Control of the band gap and control of the electron density are required for obtaining such an electronic element. The high electron-mobility transistor is obtained by laminating zinc oxide semiconductors having different magnesium concentrations one another as described in patent document 1.
The zinc oxide transistor used for the electronic element is a thin film crystal of zinc oxide usually manufactured by a thin film process such as a sputtering method, CVD method, pulse laser vapor deposition method and molecular beam epitaxial method (for example, non-patent document 2 and non-patent document 3). The thin film crystal of zinc oxide obtained by a thin film synthesis method is particularly grown in a high non-equilibrium environment, which is different from a chemical vapor transfer method shown in non-patent document 2 and a hydrothermal synthesis method shown in patent document 3.
Accordingly, as described in non-patent document 2, growth temperatures and oxygen partial pressures serve as parameters for growing the thin film crystal in a growth vessel for allowing the crystal to grow, and lattice parameters and electron densities of the thin film crystal change in a complicated manner by these parameters. In other words, different from zinc oxide manufactured in an environment at a high temperature under a relatively equilibrium state, electronic characteristics and optical characteristics of the zinc oxide semiconductor material obtained are not uniquely determined simply by its chemical composition in the zinc oxide semiconductor manufactured under a non-equilibrium condition.
However, no instructions considering the production under such non-equilibrium state are given on the structure of the electronic element in the invention shown in patent document 1 with respect to selection of the zinc oxide semiconductor material. In addition, development of an effective technology is required for the production method of the zinc oxide-based electronic element by taking the non-equilibrium state into consideration.
A thin film material concomitantly realizing a high charge density and high charge mobility as a result of charge transfer has been provided in a gallium arsenide-based thin film substrate formed by alternately laminating the charge transfer layer and charge-supplying layer as shown in non-patent document 1. On the other hand, it has been considered to endow the zinc oxide thin film with high conductivity by giving a charge separation state as shown in non-patent document 4.
While a super-lattice structure is formed by alternately depositing a thin film layer of a zinc oxide solid solution doped with magnesium and aluminum, and a thin film layer of pure zinc oxide as described in non-patent document 4, no improvement of electron mobility is attained as expected by realizing charge transfer. This is because the lattice constant varies in a complex manner when a solid solution in which both magnesium and aluminum are dissolved in zinc oxide is formed as described below.
Patent document 1 shows an art for changing the band gap of zinc oxide by adding magnesium. Actually, a decrease of the lattice constant as a result of simply adding magnesium is recognized in the solid solution of zinc oxide (Zn, Mg)O. In non-patent document 4, it is attempted to form zinc oxide with high mobility by taking advantage of the change of the lattice constant, and to reduce the lattice constant by substituting zinc with a cation having a smaller ionic radius than zinc such as magnesium, while the thin film of the zinc oxide solid solution, in which zinc is substituted with a donor-forming cation such as aluminum, is intended to be used as a layer for supplying electrons.
However, this attempt failed as described in non-patent document 4. This means that the zinc oxide laminated structure in which charge separation state is realized as an object of the present invention is not obtained in a construction comprising the charge-supplying layer and charge-receiving layer and in the absence of an electric field applied between the charge-supplying layer and charge-receiving layer, by a simple instruction that magnesium is only added as an element for changing the lattice constant and for forming a donor.
Non-patent document 5 shows that, since the band gap of zinc oxide is changed by adding magnesium. This effect causes formation of a multiple quantum well structure to permit luminous efficiency of excitons to be enhanced. However, according to non-patent document 5, while a laminated structure of two kinds of zinc oxide having different band gaps one another is obtained, the charge density in the thin film is not controlled, and a charge separation state as an object of the present invention has not been attained.
A spontaneous super-lattice structure is obtained by adding indium to zinc as described in non-patent document 6 or in patent document 4. This super-lattice structure has a structure that may be assumed to be a laminated structure of an indium oxide layer and a zinc oxide layer, and is represented by a chemical formula In2O3(ZnO)m where m is an integer.
While this laminated structure has a super-lattice structure based on the crystal structure of zinc oxide, any periodically introduced In2O3 layers function as neither a carrier-supplying layer nor a carrier-accepting layer, and an insulating material is obtained when high crystallinity and low defect concentration are realized. Accordingly, the super-lattice structures shown in these literatures cited above are not considered to be the zinc oxide-based laminated structure having the charge separation state as the object of the present invention.    Non-patent document 1: R. Dinger, H. L. Stormer, A. C. Gossarland and W. Wiegmann, Applied Physics Letters, vol. 33, p 665, 1978    Non-patent document 2: Ohgaki, T., Ohashi, N., Kakemoto, H., Sawada, S., Adachi, Y., Haneda, H. and Tsurumi, T., Journal of Applied Physics, vol. 93, No. 4, p 1961-1965, 2003    Non-patent document 3: Ogino, T., Komatsu, M., Sakaguchi, I., Hishita, S., Ohnishi, N., Takenaka, T., Okiku, Kawamoto, N. and Haneda, H., Key Engineering Materials, vol. 181-1, p 101-104, 2000    Non-patent document 4: 15th Autumn Symposium, Japan Ceramic Association, Lecture No. 2J15    Non-patent document 5: Ohmoto, A., Kawasaki, Y., Koida, T., Masubuchi, K., Koinuma, H., Sakurai, Y., Toshida, Y., Yasuda, T. and Segawa, Y., Applied Physics Letters, vol. 72, No. 19, p 2466-2468, 1998    Non-patent document 6: Ohashi, N., Sakaguchi, I., Hishita, S., Adachi, Y., Haneda, H. and Ogino, T., Journal of Applied Physics, vol. 92, No. 5, 2378-2384, 2002    Non-patent document 7: G. H. Jenden and T. Skettrup, Phys. Status, Sold (b), vol. 60, p 169, 1973    Non-patent document 8: N, Ohashi, T. Ishigaki, N. Okada, H. Taguchi, I. Sakaguchi, S. Hishita, T. Sekiguchi and H. Haneda, Journal of Applied Physics, Vo. 93, p 6386, 2003    Patent document 1: Japanese Unexamined Patent Application Publication No. 2003-046081    Patent document 2: Japanese Unexamined Patent Application Publication No. 5-70286    Patent document 3: Japanese Unexamined Patent Application Publication No. 7-242496    Patent document 4: Japanese Unexamined Patent Application Publication No. 2003-041362